The second series of Miranda is being shown again on BBC One tonight at
8.30pm. Read Chris Harvey's interview with the queen of comedy

What is Miranda Hartreally like? It is an inevitable question when you play a character called Miranda in a sitcom named after yourself. The on-screen Miranda has endeared herself to millions as the daffiest of heroines, enjoying nights in with friends made of fruit, singing a duet with a vacuum cleaner made up to look like a man, and finding it impossible to walk past a coat stand without falling over. Today, the real Miranda is having her photograph taken next to a swimming-pool in a garden in west London; and one of her entourage confides a secret about her: 'She's just like she is on TV, that's her.'

So where's the dividing line?

'My question to myself is whether to reveal that or not,' Hart says, later. 'I like the idea of people not knowing where the line is. That's really annoying, isn't it?'

It's not going to become any easier to work out from the book Hart has just written, her first. Is It Just Me? includes advice on 'vital subjects' such as beauty, technology and weddings, according to 'the wonderful laws of Miranda-land', plus all sorts of stories and anecdotes, such as the time she went to a disco, power-dressed, Dynasty-style, with bread rolls in place of shoulder pads. The only thing is, it's hard to tell which stories are entirely true, and which have been given a Miranda makeover for comic effect.

She is not wearing shoulder pads today. She is on a break between shooting the new series of Call the Midwife and the beginning of filming on the third series of her sitcom – which for the first time will be going out on BBC One. She is dressed down in jeans, blouse and some rather scary studded shoes, and seems relaxed as she chats in the garden, breaking off mid-sentence at one point when a cat wanders by. 'Hello…' she says sweetly. 'Sorry, obsessed with animals, can't not.'

Hart's book contains a running conversation with her younger self, the feisty Little M, which is a dialogue she says she has in real life. 'When I first hosted Have I Got News For You,' she says, 'it dawned on me that if 18-year-old me was sitting there, she would think, "What an extraordinary thing you've just done." It's so easy to just fear every day or fear the future, but I can go, "Well, look what we've done today, you never would have thought it." '

What was the much younger Miranda like?

'I asked my mum this, actually, because I've got a really bad memory, and she said I was quite serious. She said she'd take me to the theatre and I'd be stony-faced and she'd think, "Oh, dear," and then she'd say, "What did you think of that?" and I'd say, "It was the most brilliant and funny thing I've ever seen." She had to struggle not to laugh because I was boot-faced throughout, which makes me laugh now, so yes, I was really quite serious and probably quite anxious and shy.

'Not all the time, though,' she adds. 'I had lots of energy. I liked to go on adventures, rolling in mud and being outdoorsy. I was a bit of a tomboy, always climbing trees… Dens, I remember dens.'

Hart's rise to fame, when it happened, was quite sudden, yet it was anything but overnight. She spent years plugging away on the comedy circuit, performing in pubs, taking her one-woman show to the Edinburgh Fringe, and playing bit-parts in TV comedies, such as The Vicar of Dibley and Absolutely Fabulous, while working in a succession of temp jobs.

She took the idea for her own series to the BBC in 2003. In 2007 it became the radio show Miranda Hart's Joke Shop, before finally being commissioned for TV. The first series of Miranda aired in late 2009, and picked up a raft of British Comedy Awards – a wildly popular second series followed a year later. In 2011 she played Chummy Browne (short for Camilla Fortescue-Cholmondeley-Browne) in Call the Midwife and helped the drama about life in postwar east London to outstanding ratings. Had she always been confident that she would make it in the end?

'There was just enough belief to keep plugging away,' she says. 'Sometimes I'd get worried that I wasn't on the right path and it wasn't going to happen, because I was getting older, but I was very lucky that at 33 I got one of the leads in Hyperdrive [a sci-fi comedy that went out on BBC Two]. Thirty-three was pretty old to get your first break, but I was still young enough not to go, "This is embarrassing now", and that was when I was able to give up temping.' She says she sometimes misses the camaraderie, and 'having the boss to be naughty around'. It gave her 'heaps of material'.

Her own sitcom has been described as a return to the feelgood, cynicism-free comedy shows that were popular in the 1970s. In an age of mean, cruel comedy, hers has an innocent charm. Yet she has worked with some of the darker lights of British comedy, such as Julia Davis in Nighty Night. Does her own work have darker, edgier roots? 'I think mine's always been fairly silly,' she says. 'I do find grittier things funny but if I had to choose I would go with the big shiny floors.'

Slapstick features heavily in her series. 'Weirdly, when I wrote the first series I wasn't thinking "this character's going to fall over a lot", it just happened naturally in rehearsals,' she says. 'I didn't have a mission to put slapstick on telly, it just felt right for the character. If people think slapstick's uncool, I defy them, if they see someone falling over in the street, as long as they don't hurt themselves, not to laugh. It's a very odd person that doesn't find that funny, I think.'

She says she never really hurts herself – 'No, I just go for it like a child. I think I'm quite elastic, but I'm covered in bruises' – but admits to being clumsy in real life. 'Well, I was. I'm better now. Or I don't care now. I don't notice it as much. But I was as a teenager. My limbs were too long.'

She was actually rather sporty, a star player in the lacrosse team at the independent boarding school Downe House in Berkshire, which Kate Middleton briefly attended. Clare Balding was in the year above. Was she better at lacrosse than Balding? 'Of course I was!' she says. 'Ask her if she had Berkshire trials.'

Hart and her sister, Alice, were sent to boarding school when their father, David Hart Dyke – the commanding officer of HMS Coventry when it was sunk during the Falklands War, and later an aide de camp to the Queen – was posted to Washington, DC. Growing up, she was never aware of how important his job was. 'Not at all. He must have been away quite a lot, but he was just dad at home. Very unassuming and kind and lovely and homely. He's not Mr Naval Captain, he's not Captain von Trapp. There were no whistles.'

On her mother's side, Hart's family includes her grandfather Sir William Luce, who was the governor of Aden, and her uncle Richard, Baron Luce, who was a Conservative MP and a minister under Margaret Thatcher. 'You might say, "That's a very posh background" but it was a very un-grand, un-rich upbringing. Very normal.'

Hart is the elder sister by three years. 'I look much younger,' she says, and adds, 'I'm just saying that because it'll really annoy Alice.'

I ask whether there's anything of Alice in Stevie, the character with whom she runs a shop in her sitcom – the fictional Miranda's childhood friend. 'When I was writing the pilot, Stevie was very serious, wore a business suit, completely different to how she is now, but when I got to know Sarah [Hadland, who plays the character] we instantly became good friends, so I wanted Miranda and Stevie to be sisterly and teasing and loving. And I have that kind of relationship with my sister, like best friends. So there is a little bit.'

And what of Tilly – was there a real-life model for the blond ex-schoolfriend who delights in embarrassing Miranda at every opportunity? 'No, not a specific one. If anything Tilly came from after school; she's the PR girl who went to boarding school. I didn't have a school nemesis. I was friends with everyone at school. I mixed in with the gangs.'

Hart studied politics at the University of the West of England, in Bristol – not to follow in any family tradition, she says, but because she couldn't admit she wanted to go into comedy. In fact, she wanted to be part of a double act. 'I was just obsessed with Morecambe and Wise and French and Saunders so I thought that would be the funnest and best way. I did have a double act for a bit in Edinburgh and then performance-wise we just wanted different things. It would be nice to equally share the burden but those double-act relationships are so hard to find, very rare.'

There was a short period after university when she retreated to her parents' home – it was later reported that she suffered agoraphobia and hardly left the house for six months. 'I don't think I was feeling anything unusual,' she says now. 'I think most people in their teens or 20s will have had something similar, of just feeling a bit lost and kind of "I can't be bothered to do the world". It was a bit like, "I was having fun at university, I'm not ready to go to London and work in an office."'

She once said that pessimism is a default setting for her. 'It's getting much better,' she says now, 'but I have to force myself every day to not do glass half-empty. That makes me sound gloomy. I'm not gloomy. But I'm, "Oh, I'll never get it done," all dramatic, rather than, "Oh, I've got a third of it done and that's really good." But I suppose that kind of negativity pushes you on.'

She finds writing 'quite boring, I prefer being with people and acting', and has put pressure on herself writing the third series of Miranda, because she knows there'll be new people watching. 'I'm never happy until I get it in front of an audience and know whether it's funny, but there's a couple of episodes that I think are right. Until we're rehearsing it and performing it and hear the laughter, you never know.'

Does she have an outlet for her silliness and naughtiness in her everyday life?

'There are times when it comes out,' she says. 'I very nearly threw my publicist in the pool earlier.'

Because…

'Because it would have been funny. In fact, had I not been in a work situation, I would have dived on that swan [there is an enormous inflatable one in the swimming-pool], and then been dripping wet all day. I have to go to the BBC after this, and it would have been really awkward… "She jumped on a swan, she's nearly 40…" But in the moment, diving on to the swan would be worth it, you hope. I think I'm always predominantly guarded enough or middle-class enough to stop myself – British enough, perhaps – but as I get older, I'll just get worse. I'll probably be really embarrassing when I'm 70. I just won't care at all.'

Her character in the sitcom has become almost a poster girl for the single woman who is hopeless when it comes to men. Does it define people's perception of her?

'Obviously she's a character I am playing, she's a clown. I suppose people see the sitcom persona and they might wonder how much of that is me. Through my twenties I was predominantly single, so that was a big influence when I was writing comedy. I don't know what people think of me, hopefully not the batty spinster with the dog! I would like to get married, but I'm quite good at taking a day at a time and just seeing what the job throws up, because the job is interesting from that point of view, you never know what's going to happen next, and it is all different, every day.'

Is she someone who falls in love easily? 'I'm very romantic. I see the romance in every situation but probably don't fall in love easily, no. I think it's a wonderful pure thing, so I want to make sure it's right,' she says, then looks surprised at herself for saying it. Is she in love with anybody at the moment? 'Hmmm,' she ponders. 'Maybe. Maybe not. That's it. That's all I'm saying.'

Fame has made her want to be more private. 'I'm embarrassed about telling people too much, you want to be able to shut your door. It's a weird experience. There's been a photographer sitting outside my house for a while on and off and that's very odd. I don't think of myself as someone famous.' It also puts pressure on her to behave in certain ways. 'I don't like the idea of looking grumpy, or if a kid who likes the show comes up I like to be smiley and open to them. It leaves less room to be silent socially. I'm not one of those people who has to be "on" and has to talk all the time… I feel I don't want to disappoint people who might want a laugh.'

I wonder if being in the public eye has put pressure on her to conform to the rules of celebrity – to dress a certain way, to lose weight… 'Oh gosh no, not at all,' she says. 'I don't feel any pressure to look a certain way at all. It is hard because you see images of yourself in this job that people don't normally. And you're confronted with the fact that you've got vast thighs and enormous upper arms. But that's where being a character actor is great because I don't have to be the leading lady. Who'd want to be the beautiful one?

'I lost some weight last year, and put it back on this year because I've been sedentary during the writing process. In my personal life, I thought, "Oh that's annoying," because I was getting fit and slim, and then I thought actually people would think that just because the show had done well, I would have lost weight to go on TheGraham Norton Show, and then it might have jarred, so I thought maybe it's a blessing in disguise that I put the weight back on.'

Hart will go back to finish filming the new series of Call the Midwife in November. She still can't quite believe the level of its success. She got the part when the author of the original memoir, Jennifer Worth, contacted her by letter. 'She sent me the book, and said if it gets made into a telly show, I'd love you to be Chummy because you remind me of her. She'd pencilled in the index the page numbers where Chummy appears, so I went straight there and thought, "Oh my goodness, I've got to play her." But at that point I didn't think it would happen.'

Will we see her in more serious roles? 'I hope so. Chummy is just perfect for me because it's another fish out of water, and it enabled me to do some more emotional stuff. But I'd love to do different things as well.'

'Is It Just Me?' by Miranda Hart, (Hodder & Stoughton, £20) is published on October 11 and can be pre-ordered for £18 plus £1.35 p&p from Telegraph Books (0844-871 1515; books.telegraph.co.uk). Read an extract from the book in 'Stella' on Sunday 23 September